In the known devices of this type, an air intake chamber is usually provided, which can be fed selectively either with external air taken in from outside the passenger compartment or with recirculated air taken up from the passenger compartment.
A blower is provided at the outlet from the air intake chamber for blowing the airflow so as to process it (by heating and/or cooling) before sending it into the passenger compartment through various appropriate nozzles.
In general, the air intake chamber houses a flap which controls the external air intake and the recirculated air intake selectively -Ln order to send into the passenger compartment either a flow of external air or a flow of recirculated air, depending on the air temperature regarded as comfortable by the occupants of the vehicle.
Hence, when the vehicle is passing through a polluted area, it is recommended that the device be placed in "recirculated air" position, so that the passenger compartment is supplied with recirculated air, and not with external air.
Conventionally, the change from the "recirculated air" position to the "external air" position, or vice versa, takes place at the discretion of the driver of the vehicle, that is to say on the basis of subjective observation.
It is also known to place a pollution sensor, also called air quality sensor, on a vehicle in order to detect the presence of a pollutant and then place the device in one or other of the "external air" and "recirculated air" positions, either at the initiative of the driver or automatically.
Up to the present time, these pollution sensors have been installed either at the front of the vehicle (behind the bumper, for example, or near the engine cooling unit), or in the "water separator" which is situated at the foot of the windshield.
In any event, these sensors were placed upstream of the air filter which serves to trap various impurities (particles, dust, etc.) or various gases (hydrocarbons, nitrogen dioxide, sulfur dioxide, etc.) contained in the external airflow.
Because the pollution sensor is always situated upstream of the air filter, the result is that this sensor is exposed to a flow of air loaded with particles, dust and various pollutants. This has the drawback of leading, in the more or less long-term, to fouling and to rapid deterioration of this type of sensor.